The accumulation of inorganic mineral scales in oil field formations and production equipment is a major problem for the oil industry. Deposition of inorganic mineral scale in oil-bearing formations and on production tubing and equipment causes significant and costly loss of production. The primary offenders are carbonates and sulfates of calcium, barium and strontium. These compounds may precipitate as a result of changes in pressure, temperature and ionic strength of produced fluids or when connate reservoir waters mix with injected waters during secondary recovery operations. In order to avoid costly losses in production or post-scale treatments it is necessary to prevent deposition of barium sulfate scales downhole as well as in post production processing.
For more detailed discussions of the scaling problem in oil field production systems, reference may be made to Johnson, "Water Scaling Problems in the Oil Production Industry", Spec. Publ., Roy. Chem. Soc., 1983 (45), 125; Weintritt, et al., "Unique Characteristics of Barium Sulfate Scale Deposition," Petroleum Transactions, J., Pet. Tech., 1381, October, 1967; Vetter, "How Barium Sulfate Is Formed: An Interpretation" J. Pet. Tech., 1515, December, 1975; Leung, et al., "Nitrilotri(Methylenephosphonic Acid) Adsorption on Barium Sulfate Crystals and Its Influence On Crystal Growth", J. Crystal Growth, 44, 163 (1978); Templeton, "Solubility of Barium Sulfate in Sodium Chloride Solutions from 25.degree. to 95.degree. C.", J. Chem. Eng. Data, 5, 514 (1960); and, Nancollas, et al., "Crystal Growth and Dissolution of Barium Sulfate", Soc. Pet. Eng. J., 509, December, 1975.
Barium and strontium sulfate scales are of particular concern because of their extremely low solubilities (10.sup.-4 to 10.sup.-5 Molar [Ba.sup.++ ] depending upon brine concentrations and temperature). At room temperature the solubility of BaSO.sub.4 in distilled water is about 2 ppm and at 80.degree. C. is about 4 ppm. In 0.5M NaCl, the solubility is 7 ppm at room temperature and about 30 ppm at 80.degree. C.; in 1.0M NaCl, about 23 and 42 ppm, respectively.
While effective measures have been developed for the removal of calcium carbonate and calcium sulfate scales, barium sulfate scaling is difficult to treat once it develops. Effective scale control measures for BaSO.sub.4 are important in the North Sea and in other areas as well. Water from the North Sea contains 2,500-2,700 ppm sulfate, while formation waters from that area analyze as high 250 ppm barium.